1. Field of the Invention
The present invention concerns CVD apparatus, and in particular CVD apparatus which use plasma and which are suitable for depositing films on large flat panel substrates.
2. Discussion of Related Art
The use of high temperature polysilicon type TFT (thin film transistors) and the use of low temperature polysilicon type TFT were known in the past as methods for the production of liquid crystal displays. A quartz substrate which can withstand high temperatures of 1,000 C or above is used for obtaining a high quality oxide film and oxide film interface with polysilicon in the method of production where high temperature polysilicon type TFT is used. On the other hand, a glass substrate as generally used for a TFT is used in the production of low temperature polysilicon type TFT liquid crystal display and so film deposition must be carried out in a low temperature environment, such as at 400 C. The production of liquid crystal displays using low temperature polysilicon type TFT has the advantages of not requiring the use of a special substrate and so this method has come into practical use in recent years and the amount of production continues to expand.
Plasma enhanced CVD is used in those cases where an appropriate silicon oxide film is deposited at a low temperature as a gate insulating film in the production of a liquid crystal display using low temperature polysilicon type TFT. Silane and tetraethoxysilane (referred to hereinafter as TEOS) are used as precursor gases when depositing silicon oxide films using plasma enhanced CVD.
When a silicon oxide film is deposited using plasma enhanced CVD with TEOS as the precursor gas, the precursor gas is delivered directly into the plasma which is formed in a conventional plasma processing apparatus. The precursor gas and oxygen react vigorously and the reaction product is formed in the gas phase. The reaction product forms dust particles which cause the formation of defects in the TFT elements. There is a problem in that the yield is reduced by the formation of dust particles. Moreover, there is also a problem in that the film properties are adversely affected by the implantation in the silicon oxide film of the high energy ions which are present in the plasma which is in contact with the substrate.
Plasma enhanced CVD apparatus in which a remote plasma system is used has been suggested in the past as a means of resolving the abovementioned problems. In a remote plasma system, the region in which the plasma is generated and active species such as radicals are formed is separated from the substrate in the plasma processing apparatus and the precursor gas is delivered close to the region in which the substrate is disposed. The radicals which are formed in the plasma region are diffused toward the region in which the substrate has been disposed and reach the space in front of the surface of the substrate which is being processed. The vigorous reaction between the plasma and the precursor gas is suppressed in a plasma processing apparatus of the remote plasma system and there is little dust particle formation, and there is a further advantage in that the implantation of ions into the substrate is also suppressed.
However, the plasma generating region and the region in which the substrate is disposed are formed separately with an intervening Joining space in a plasma processing apparatus of the remote plasma system. The radicals which are produced at a location remote from the substrate are delivered to the substrate by a diffusion process through the joining space. The rate of film deposition is reduced in a remote plasma system and there is a further problem in that the distribution in the vicinity of the substrate surface is poor. In particular, problems arise in that the system cannot be applied to the substrates which have a large surface area which are used for large liquid crystal displays because of the poor distribution in the vicinity of the substrate surface.